In the US, direct-seeded rice is grown in sensitive aquatic habitats. Because of this, concerns have been raised on the environmental safety of herbicides used on rice fields, and there is a need for safer and more effective rice herbicides both in conventional and organic rice growing systems. California farmers, who have faced product restrictions due to water quality concerns, report costs of $150/acre for herbicide treatments compared to $35-70/acre for other crops. To date, no selective herbicides are available to control weeds in organic rice. Both broadleaf and grassy weeds in organically grown rice can only be controlled through water management and hence, yield loss in organic rice due to weeds can exceed 50%.
Natural products are substances produced by microbes, plants, and other organisms. Microbial natural products offer an abundant source of chemical diversity, and there is a long history of utilizing natural products for pharmaceutical purposes. However, secondary metabolites produced by microbes can also be successfully used for weed and pest control in agricultural applications.
The best known herbicide derived from a microbial natural product is glufosinate, the synthetic version of phosphinothricin, a breakdown product of bialophos produced by Streptomyces viridichromogenes and Streptomyces hygroscopicus (Duke, Dayan et al. 2000) and (Hoagland 2001). Hydantoicidin, a nucleotide analog derived from a particular strain of Streptomyces hygroscopicus has been the subject of structure-activity and patenting work by agrichemical companies but no commercial products are on the market. (Duke, Dayan et al. 2000). Cornexistin, a nonadride phytotoxin from the basidiomycete Paecilomyces variotii has good herbicidal activity against monocotyledonous and dicotyledonous weeds. It appears to be a proherbicide, being converted in vivo to an inhibitor of at least one isozyme of aspartate aminotransferase. (Duke, Dayan et al. 2000). Recently, Dow AgroSciences found a compound from two species of fungi isolated by the company Mycosynthetix, a Fusarium species and Nodulosporum species. The broad spectrum, systemic compound, mevalocidin, kills weeds in 3-4 weeks (Gerwick, (Graupner et al. 2005).
Thaxtomins (4-nitroindol-3-yl-containing 2,5-dioxopiperazines) are a family of dipeptide phytotoxins produced by plant-pathogenic Streptomyces sp. (S. scabies, S. acidiscabies) that cause scab diseases in potato (Solanum tuberosum) (King, Lawrence et al. 1992). Toxin production occurs in diseased tissue and can also be elicited in vitro in an optimal growth medium containing oat bran (Loria, Bukhalid et al. 1995; Beauséjour, Goyer et al. 1999). King and her coworkers (King, Lawrence et al. 2001) demonstrated that all plant pathogenic species in the Streptomyces family produce one or more thaxtomins with herbicidal activity. Hiltunen et al. (Hiltunen, Laakso et al. 2006) purified four thaxtomin analogs (thaxtomin A, thaxtomin A ortho isomer, thaxtomin B and thaxtomin D) from cultures of S. scabies and S. turbidiscabies and showed that all four compounds induced similar symptoms of reduced shoot and root growth, root swelling. (at 10-200 ppb) and necrosis (at 200-1000 ppb) on micropropagated in vitro cultures of potato. In addition thaxtomins applied in combinations, showed additive effects, but no synergism (Hiltunen, Laakso et al. 2006). According to Duke et al. (Duke, Baerson et al. 2003) both thaxtomin A (FIG. 1) and thaxtomin D have marked activity as pre and post emergent, non-systemic herbicides, and concentration of less than 1 uM of thaxtomin A causes cell swelling, necrosis and growth inhibition in mono and dicotyledonous seedlings (Healy. Wach et al. 2000). Thaxtomin has been evaluated as an herbicide by Dow Agro Sciences, Inc., and while active, it lacked systemic action (King, Lawrence et al. 2001). The presence of a nitro group in the indole ring required for an L,L-configuration of the diketopiperazine appears to be the minimal requirement for phytotoxicity. Position of nitro group in the indole ring is very site specific, and the phenyl portion of the phenylalanine plays a necessary role in structural requirements of phytotoxicity (King, Lawrence et al. 1989; King, Lawrence et al. 1992; King, Lawrence et al. 2003). The herbicidal mode of action is based on disruption of cell wall synthesis (Fry and Loria 2002), with inhibition of cellulose biosynthesis being the main target (King et al., 2001; Duval et al., 2005; Johnson et al. 2007). Recently, Kang et al. (Kang, Semones et al. 2008) have described the use of thaxtomin and thaxtomin compositions as algaecides to control algae in water environments.